The present invention relates to a homogeneous process for the hydrogenation of carboxylic acids and/or derivatives thereof. More particularly it relates to a homogeneous hydrogenation process which can be carried out in the presence of water.
Many catalyst systems are known which are suitable for use in the hydrogenation of carboxylic acids, acid anhydrides, esters or amides. Traditionally such reactions are carried out using heterogeneous catalysts and often high temperatures and pressures. A disadvantage of these heterogeneous catalyst systems is that many are intolerant of acid feedstocks and therefore have limited use.
In order to overcome this problem, catalysts have been suggested for the hydrogenation of carboxylic acids and their derivatives based on ruthenium/phosphine systems. Examples of these catalyst systems include those described in U.S. Pat. No. 5,047,561, U.S. Pat. No. 5,079,372, U.S. Pat. No. 5,580,991, U.S. Pat. No. 5,077,442, U.S. Pat. No. 5,021,589, U.S. Pat. No. 4,931,573, U.S. Pat. No. 4,892,955, “Hydrogenation reaction of carboxylic anhydrides catalyzed by a new and highly active cationic ruthenium complex”, Y-Hara et al Chem Lett (1991) 553, U.S. Pat. No. 3,957,827, U.S. Pat. No. 4,485,245 and U.S. Pat. No. 4,480,115 which are incorporated herein by reference.
However, whilst the systems described in these document provide processes which in general adequately enable hydrogenation reactions to be carried out, they do suffer from certain disadvantages and drawbacks. In particular, they require that the hydrogenation reaction is carried out in the absence of water since it is believed that any water present inhibits the catalyst or significantly reduces the rate of reaction. For example, in U.S. Pat. No. 5,047,561 an organic solvent is used and it is stated that the amount of water present should be controlled and should be no higher than 1% by weight. In “Hydrogenation reaction of carbonyl compounds catalyzed by cationic ruthenium complexes”, H-Inagaki et al, Science and Technology of Catalysis (1994) 327 it is explained that the presence of water retards the hydrogenation reaction of succinic anhydride in the presence of a ruthenium trialkyl phosphine complexes in the presence of a promotor and that it is necessary to remove the water produced by hydrogenation in the gas stream and in U.S. Pat. No. 3,957,827 and U.S. Pat. No. 4,485,245 scavengers are used to remove any water produced in the reaction with the aim of improving yield and productivity.
Many of these known catalyst systems also require the presence of a promotor to increase the selectivity and activity of the ruthenium catalyst. Examples of this include U.S. Pat. No. 5,079,372 and U.S. Pat. No. 4,931,573 where reactions are carried out in the presence of an organic solvent and a metal selected from Group IVA, VA and III is required as a promotor.
Another example of the use of a promotor may be found in U.S. Pat. No. 5,077,442. In this case a phosphorous compound is used to promote selectivity and conversion. This document teaches that any water produced in the reaction is removed from the reaction zone as the presence of water is said to decrease selectivity and conversion.
Another suitable promotor described is a conjugate base of an acid and in this connection reference may be made to U.S. Pat. No. 5,021,589 and U.S. Pat. No. 4,892,955. In this latter case, it is noted that components of the catalyst system are susceptible to hydrolysis under the reaction conditions and that a hydrogen purge was required to remove water produced during the reaction.
Whilst these processes go some way to providing adequate catalyst systems, there is still a need for alternative process which allow for efficient hydrogenation of carboxylic acids and/or derivatives thereof with good conversion and selectivity to the desired products. Surprisingly, we have now established that the presence of water is not only not disadvantageous but indeed offers positive advantages.